Bolted-plate towers are preferred to welded towers because the plates can be given a protective coating in the factory, i.e., before the tower is erected. The plates can be coated with a thin layer of glass, for example.
A problem with bolted-plate towers is in the precautions that must be taken to prevent leaks. Jointing compound is daubed over all the surfaces that are to be squeezed together by the bolts; and if the bolts are tightened carefully then the bolted joints are secure enough. But the edge-abutment joints, where they occur between the flange-less plates, cannot be pressed together by tightening the bolts.
The tower is designed and built so that the abutting edges of plates are as close together as possible. However, the abutting edges cannot be allowed to actually come in contact with each other, because such contact might damage the delicate coating.
U.S. Pat. No. 2,729,313 (ERNESTUS, Jan. 3, 1956) shows an example of a bolted-plate tower made of glassed, flangeless plates, and shows the presence of such edge-abutment joints.
Typically, it is possible to arrange that the abutting edges can never be closer together than about 1 mm, but yet never more than about 4 mm apart (i.e., there is a tolerance on the gap of 3 mm). If the plates were to be made with a tighter tolerance on the gap than that, then they could not be manufactured economically on normal press machinery.
Thus the designer of the tower is faced with the fact that some of the abutment gaps might be as large as 4 mm. The designer must therefore limit the height of the tower so that the water pressure at the very bottom is not enough to cause the jointing compound to be extruded through a gap that is 4 mm wide.
However, if the tolerance on the gap between abutting plates could be reduced from 3 mm down to say 1 mm (so that the gap varied from 1 mm to only 2 mm) then water towers could be built to a greater height.
The invention provides a means whereby the abutment gap can be economically controlled within tight limits, to permit higher towers to be built.